This invention relates generally to holographic elements, and more particularly to the manufacture of holographic elements that are highly efficient and have a large bandwidth. These holographic elements are especially well suited to be used as solar concentrators and are most effective as part of solar concentrating systems.
Holography is a technique for recording and later reconstructing the amplitude and phase distribution of a coherent wave disturbance. Generally, the technique utilized for producing a holographic element is accomplished by recording the pattern of interference between two optical beams or waves. These waves, one for example, reflected from an object, usually called the object wave, and the other which by-passes the object, usually called the reference wave, strike a light-sensitive recording medium such as a holographic film or plate. Thus incident on the recording medium is the sum of the light from the object and the mutually coherent reference wave.
While all light sensitive recording media respond only to light intensity, in the pattern of the interference between the two waves there is preserved a complete record of the amplitude and also the phase distribution of one of the waves with respect to the other. Amplitude information is preserved as a degree of modulation or in the depth of the interference fringes, while phase information is preserved as variations of the positions or pattern of the fringes.
Although a holographic element is recorded utilizing one wavelength (created by a laser) it can diffract either a narrow or broad bandwidth depending upon the fabrication parameters of the holographic element. In the case of a broad bandwidth holographic element on playback, other wavelengths will form a spectrum of wavelengths around the recorded wavelength. Unfortunately, when utilizing conventional holographic elements for concentrating visible light (where the wavelengths differ by a factor of 2) much of the original light is lost. In fact, it is typical for conventional holographic elements to have expected efficiencies of only 10% when diffracting or concentrating across a broad bandwidth. This presents particular problems when attempting to utilize holographic elements for solar concentrators.